mfd: omap-usb-host: Add device tree support and binding information

Allows the OMAP HS USB host controller to be specified
via device tree.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>